1. Field of the Invention
The present invention relates to a deformed folded dipole antenna in which two parallel sections coupled through short sections are deformed into U-shapes opposed to each other. The present invention also relates to a method of controlling an impedance of a deformed folded dipole antenna and an antenna device including a deformed folded dipole antenna.
2. Description of the Related Art
As an example of a folded dipole antenna, JP-A-2005-260567 discloses a deformed folded dipole antenna.
The deformed folded dipole antenna includes a pair of parallel sections (side portions 9, 12 and side portions 10, 13 in FIG. 1 of JP-A-2005-260567) arranged in parallel with each other and short sections (folded structure 11, 14) respectively coupling ends of the pair of parallel sections. One of the parallel sections (side portions 9, 12) has a feeding point at a middle point of an electric length in a longitudinal direction.
The other parallel section (side portions 10, 13) without a feeding point has a U-shape including a pair of opposing side portions opposed to each other and a connecting side portion (a portion between the folded structures 16, 18) connecting ends of the opposing side portions.
The parallel section (side portions 9, 12) having the feeding point includes two L-shape sections. One (side portion 9) of the L-shape sections is arranged in parallel with a part of the connecting side portion and one of the opposing side portions (side portion 10). The other (side portion 12) of the L-shape sections is arranged in parallel with a part of the connecting side portion and the other of the opposing side portions (side portion 13).
In the two L-shape sections (side portions 9, 12), portions (portions between the folded structures 15, 17) opposed to the connecting side portions (portions between the folded structures 16, 18) are opposed to each other at a predetermined distance therebetween and are arranged in the same straight line with each other. Accordingly, the two L-shape sections form a cut U-shape. The feeding point is provided at end portions of the L-shape sections opposed to the connecting side portion.
Thus, in the deformed folded dipole antenna, the two parallel sections opposed to each other are coupled through the short sections, one of the parallel sections has the U-shape, and the other of the parallel sections has the cut U-shape.
When a width of each parallel section is uniform throughout a longitudinal direction, an impedance of a folded dipole antenna can be controlled by changing a ratio of a width of a parallel section having a feeding point with respect to a width of a parallel section without a feeding point as described, for example, in JP-A-2004-228917.
When the conventional impedance control method is applied to the deformed folded dipole antenna having the U-shape, a width of the cut U-shape of the parallel section having the feeding point are set to be smaller than a width of the U-shape of the parallel section without a feeding point throughout the longitudinal direction of each parallel section.
For example, when two parallel sections in a deformed folded dipole antenna have the same width, in order to increase an impedance of the deformed folded dipole antenna, the width of the parallel section including the feeding points is decreased throughout the longitudinal direction and the width of the parallel section without a feeding point is increased throughout the longitudinal direction. In the above-described case, an outside dimension of the deformed folded dipole antenna along a plane on which the U-shape are arranged depends on an outside dimension of the parallel section without a feeding point whose width is increased. Thus, the outside dimension of the deformed folded dipole antenna is increased both in a direction along the opposing side portions and a direction along the connecting side potion. Especially in a deformed folded dipole antenna in which parallel sections have U-shapes, because two opposing side portions are arranged in parallel with each other in a direction perpendicular to a connecting side portion, an outside dimension in a direction along the connecting side portion is increased by an increased amount of the widths of the two opposing side portions.
In contrast, in order to decrease the impedance, the width of the parallel section including the feeding points is increased throughout the longitudinal direction compared with the width of the parallel section without a feeding point.
For example, when two U-shaped parallel sections in a deformed folded dipole antenna have the same width, in order to decrease an impedance of the deformed folded dipole antenna, the width of the parallel section including the feeding points is increased throughout the longitudinal direction and the width of the parallel section without a feeding point is decreased throughout the longitudinal direction. In the above-described case, an outside dimension of the deformed folded dipole antenna along a plane on which the U-shape is arranged depends on an outside dimension of the parallel section having the feeding points whose width is increased. Thus, the outside dimension of the antenna is increased both in a direction along the opposing side portions and a direction along the connecting side portion. Especially in a folded dipole antenna in which parallel sections have U-shapes, because two opposing side portions are arranged in parallel with each other in a direction perpendicular to a connecting side portion, an outside dimension in a direction along the connecting side portion is increased by an increased amount of the widths of both of the opposing side portions.
In this way, when an impedance of a deformed folded dipole antenna having a U-shape is controlled so as to ensure an impedance matching with an external device such a coaxial cable and a parallel feeder line, widths of two opposing end portions in one of parallel sections are larger than before controlling impedance, and thereby the outside dimension of the deformed folded dipole antenna along the connecting side portion may be increased.
An increase of the outside dimension may also be restricted by fixing the width of one parallel section and decreasing a width of the other parallel section. However, there is a manufacturing limitation in decreasing the width. Especially, in a small antenna originally having a small width, a control range of impedance is small.